Aprochromatic

Aprochromatic is a term used in optics to describe a lens or optical system that is not achromatically corrected. Achromatism is a specific type of chromatic correction where two wavelengths of light are brought to a common focus. An aprochromatic lens, therefore, does not achieve this level of correction. In practice, most lenses exhibit some degree of chromatic aberration, which is the failure of a lens to focus all colors (wavelengths) of light to the same point. A simple lens made of a single type of glass is typically not achromatically corrected.

The degree of chromatic aberration in an aprochromatic system can vary significantly. Without specific correction, different colors will focus at slightly different distances, leading to color fringing in images, especially when viewing objects with high contrast. This effect is more noticeable at the edges of the field of view and with wider apertures. For applications where precise color reproduction and sharpness are critical, such as in high-quality photography, microscopy, or scientific instrumentation, aprochromatic lenses are generally unsuitable. Instead, more complex achromatic, apochromatic, or even superachromatic designs are employed to minimize chromatic aberration across a broader spectrum of light. The term aprochromatic is less commonly used than the direct descriptions of the aberration present, such as "uncorrected" or "simple lens."